Multi-color graphic designs are typically provided in electronic form and are called digital images herein. Such images may include text, line art, and continuous-tone regions. Printing such a digital image with a set of inks involves separating the image into a set of color separations, one separation for each printing ink. Each color separation is a monochrome image that is sequentially printed with its ink on paper or another substrate to form a print of the design. Several printing processes are known. The separations may be exposed directly onto paper or other substrate, using electro-photography or other direct processes. The separations may also be printed using offset and other plate-based printing processes, in which case, a set of printing plates are first made from the separations, and the prints made by sequentially applying each ink using the corresponding plate. The plates may be directly made using a direct imaging imagesetter, or made from film exposed in an imagesetter.
Misregistration and shifts between the separations may cause several artifacts to appear in printed images. Such artifacts include white bands appearing in the edge between areas of constant color. Spreading or choking areas of constant color to cause overlap is a known method for avoiding such artifacts. The spread itself is called a trap, and the general process for correcting for such misregistration-based artifacts is called trapping.
Methods for automatically trapping digital images are known, and these automatically determine and apply traps. Automatic trapping methods generally include the steps of 1) determining the edges between regions, 2) determining whether or not to apply traps in each such edge, and what the color and location of each trap should be, and 3) applying the traps. Automatic trapping methods fall into two general categories, pixel trapping, and object trapping, and this invention is applicable to both methods. Pixel trapping is most applicable when the images are available in the form of pixel files, while object based trapping is usually applicable to when the images are described in some object form, including in a page description language (PDL) such as a PostScript (Adobe Systems Inc., San Jose, Calif.).
Prior art automatic trapping techniques include those described in U.S. Pat. No. 4,931,861 to Tagunuchi, U.S. Pat. No. 4,583,116 to Hennig et al., U.S. Pat. Nos. 5,113,249, 5,323,248, 5,420,702, and 5,481,379 to Yosefi, U.S. Pat. No. 5,295,236 to Bjorge et al., U.S. Pat. No. 5,542,052 to Deutch et al., U.S. Pat. No. 5,313,570 to Dermer et al., U.S. Pat. Nos. 5,613,046 and 5,668,931 to Derner, U.S. Pat. No. 5,440,652 to Ting, U.S. Pat. No. 5,581,667 to Bloomberg, U.S. Pat. No. 5,666,543 to Garland, and U.S. Pat. No. 5,761,392 to Yacoub et al.
While there is much prior art in automatically determining when traps should be applied, little is known about how to automatically determine trap shapes that are desirable. A trap on an edge between two objects generally extends a distance called the “trapping distance” herein into one of the objects, depending on such criteria as the relative brightness of the objects. Thus, each point on an edge potentially generates a trap area that is at most the trapping distance away from the point. “On edge” trapping generated a trap area that truncates on the object edge, even if otherwise the trap area would extend beyond such an edge. Such “on edge” trapping avoids the trap area of an object exceeding the edge of an adjacent object.
So-called beveled trapping—also called bisector trapping—has the desirable property that trap areas are beveled between edges of objects, so that rather than a trap area truncating on an object edge, the trap area terminates on the bisector of the angle between an object edge and the trap edge (assuming equal trapping distance in all directions). That is, at begin and end areas of traps, the trap area truncates on the centerline between the edge and the trap edge.
So-called centerline trapping has the desirable property that any trap area becomes smaller as the trap area approaches other edges. A centerline trap area extends only to the lesser of the maximum distance from the edge of the trap (i.e., the trapping distance) and the centerline between the trap edge and any other edges. Beveled trapping is centerline trapping restricted to the start and end of trap areas. Beveled trapping thus generates a trap area that follows the bisector of an angle between two edges when the trap area approaches the two edges.
These concepts are now illustrated with the aid of a simple example shown in FIGS. 1A to 1D, in which any trap distances are shown exaggerated in order to illustrate features of the trap shapes. FIG. 1A shows an original image prior to trapping. In FIG. 1A, region 101 is a white background, object 103 is a yellow object, object 105 is a blue object, including a hole 107, which is a white background region. The trap area normally would be yellow and would extend a certain “trapping distance” into the blue region, resulting in a green area between objects 105 and 103. FIG. 1B shows “on-edge” trapping. The trap region 109 truncates on the edge of blue object 105. FIG. 1C shows beveled trapping. The trap region 111 follows the bisector of the angle between the trap edge and the bottom edge of object 105 to form a first bevel, and also follows the bisector of the angle between the trap edge and the left vertical edge of object 105 to form a second bevel. The border of the trap area follows the centerline in these two endpoints. FIG. 1D shows true centerline trapping. Notice that in addition to the bevels of FIG. 1C, the trap region also follows the centerline between the common edge of objects 103 and 105, and the edge of objects 105 near white background 107.
With prior art methods, to achieve any of these often desired shapes, one would need to determine the trap boundaries as some function of the distance from the edges. While on-edge-trap shapes are relatively simple to calculate, determining the centerline between edges may be computationally intense, and is needed in order to determine the trap shape to implement centerline trapping.
To further illustrate these points, FIG. 2A shows an image after beveled trapping. Yellow area 203 has an edge 204 to a darker magenta area 205, and both areas are on a white background 209. Beveled trapping generates a yellow trap area 207. Note that trap area 207 follows the bisector 211 between edge 204 and edge 208. Similarly, FIG. 2B shows the image after trapping of a light gray area 215 next to a dark green area 213 and a dark magenta area 211. Beveled trapping generates the trap areas 217 and 219, normally in light gray. Note that the bisector is followed by the trap areas close to two edges. Finally, FIG. 2C shows centerline trapping.
Presently, with prior art methods, the trap shape would need to be modified to include the bisector for beveled trapping or to follow the centerline for centerline trapping. There is a need in the art for rapidly and automatically determining trap shapes that follow the bisector.
In some applications, for example, in textile and decorative printing, different color combinations of the same design are obtained using the same separations of the design, but with different inks assigned to each separation. Such different color combinations of the same design are called colorways of the design. When trapping such a design, it is important to be able to generate trap areas that extend in both directions of an edge between areas. This enables the same trap areas to be used for different colorways. Of course this assumes the design program enables switching the color of the trap areas when the inks (or colors) assigned to the channels are changed.
It is sometimes desirable to have traps that are dependent on direction. For example, with some printing presses, misregistrations are likely to be larger in one direction than in another. It is desirable to have a trapping method that automatically generates traps that are direction dependent, that is, that extend by a different amount in each direction. Furthermore, a user should easily be able to vary the direction-dependent trapping distances.
It is also sometimes desired to be able to implement traps that gradually fade into another area rather than trap areas that have sudden edges. It is also desirable to be able to implement such “fade” trapping with a user-defined fading profile that can be easily modified, and that can be different in each direction.
While some of these features are possible using prior art trapping techniques, there is still a need for a trapping method that has one or more of the above properties, and that can be implemented on pixel files.